The invention generally relates to the correction of centering and convergence errors in cathode ray tube (CRT) displays, and more particularly to a method and apparatus for the automatic correction of such errors during normal operation of the display.
CRT displays, whether they be monitors, television receivers or projection systems, periodically require adjustments to be made to maintain proper centering of the displayed image. Color CRT displays of the type having three cathode ray beams and a screen with a mosaic of phosphor dots or stripes of recurring groups of three colors must be adjusted to maintain the convergence of the three beams over the visible surface of the screen. An analogous adjustment must be made for projection displays employing three projection CRTs. These adjustments are initially made at the factory, but with age, temperature and other environmental conditions, it is necessary to readjust centering and convergence in order to maintain the quality of the displayed image. Ordinarily this is accomplished by a skilled technician with test instruments. The test instruments used to measure convergence often resort to the use of an appliance that is placed over the CRT screen to facilitate detection of the landing point of the cathode ray beam. Such an appliance obscures the screen, and therefore these instruments are not intended to be used simultaneously with the viewing of the display. Examples of such instruments are U.S. Pat. No. 4,001,877 issued to Theodore Frederick Simpson and U.S. Pat. No. 4,035,834 issued to Anthony M. Drury.
The Simpson patent describes a test instrument that employs a photosensitive array comprising a plurality of individual photo cells, this array being placed over the CRT screen. Further, a special post-deflection coil is required to introduce magnetic fields in the region just forward of the deflection yoke to displace the scanned beams in a controlled pattern from their normal landing points on the screen. The displaced beam causes the emission of an error color, the intensity of which is measured by those photo cells which are sensitive to the error color emitted. The intensity of a reference color emitted by the phosphor deposits stimulated by the undisplace beam is then measured, and the ratio of the error color to the reference is calculated for each measurement location on the screen. The largest ratio is displayed as an indication of the color purity tolerance of the CRT. The Simpson test instrument is used primarily as a quality control device in the manufacture of color CRTs.
The patent to Drury describes a beam landing indicator for a color CRT which also employs a holder for positioning a plurality of photo cells over the screen of the CRT. While the Drury instrument does not require a special deflection coil, it does employ a special deflection generator in order to produce a clockwise rotation of the beam landing shift of the beam. This rotation is stepped in increments which occur once each vertical field of the television raster. Light variations sensed by the photo cells are combined with a reference signal to control the dot location on an oscilloscope display of the vector beam landing error. The technician can then make purity adjustments and yoke adjustments of the CRT by observing the oscilloscope display.
Automating the adjustment of color television receivers is also known. An example is described in U.S. Pat. No. 3,962,722 issued to Walter S. Ciciora. More specifically, the Ciciora patent describes a color television setup apparatus for use in a factory. Once again, a holder positions a plurality of photo cells over the CRT screen in such a manner as to obscure the view of the screen. Patterns indicative of the characteristics of contrast, brightness, color and tint are displayed on the CRT. The photo cells develop corresponding electrical signals which are supplied to circuitry that energizes a plurality of bi-directional motors that are engageable with the receiver contrast, tint, brightness and color level adjustment elements.
While the systems described by Simpson, Drury and Ciciora are useful in a factory or shop environment, what is needed is an automatic means for adjustment of convergence which is part of the CRT display. In this way, the display would be continuously maintained in proper adjustment for optimum viewing. Such a system has been provided in the above-referenced application Ser. No. 423,906 (now U.S. Pat. No. 4,485,394) filed by Ghaem-Maghami and Holshauser. According to that invention, a system is provided for the automatic correction of convergence and gray scale which employs light sensors, either singly or in an array, on or adjacent to the beam landing surface of a CRT or on or adjacent the screen of a projection receiver. The sensors can be placed proximate the overscanned area of the raster such that they are outside the normal viewing area, or in the viewing area if the sensors are made sufficiently small. In the vicinity of a sensor, two of the three cathode ray tubes or electron guns are blanked. As the light beam, in the case of a projection system, crosses the sensor, an output is produced. This output is processed to obtain accurate timing characteristics. Since the position of the sensor is known in terms of counts in both the vertical and horizontal directions, error signals can be developed by comparing the timing of the sensor output with the proper count. These error signals are used to develop vertical and horizontal correction signals to correct the convergence of the one cathode ray tube or gun. The process is then repeated for the remaining two cathode ray tubes or guns. The output of the sensor is also amplified by gated amplifiers for each of the cathode ray tubes or guns in sequential order, and the outputs of these amplifiers are compared to a preset value to develop error signals. These error signals are used to set gun drives to correct the gray scale.
The system described in application Ser. No. 423,906 (now U.S. Pat. No. 4,485,394) filed by Ghaem-Maghami and Holshauser generally operates well under most conditions; however, that system is affected by a change in picture size due to a change from a dark scene to a light scene, a change in line voltage, or a change in the blanking pulse transmitted by the television station, for example. The system is sensitive to both the amplitude and speed (slope) of the incoming light pulse. So while the system according to Ghaem-Maghami and Holshauser operates satisfactorily, it is nevertheless desirable to improve on the performance of that system.